Sion ulti half working?

I've noticed that almost every game I play sion, his ulti has some weird bug. So i stopped playing him, thinking it would get fixed, but it's still happening.

Some times when I try and use Sion ulti, it only half starts. The cursor appears infront of me, and I an't change direction. However, I only go forward at walking speed, and I don't do damage if i hit a champion, or cause the knock up or anything. I then have to press R again, but then it finally starts the speed increase, but it also instantly stops my ultimate. It happens maybe 1/5 times I do his ulti.

Has anybody else had this happen to them?